1. Field of the Invention
This invention relates to a force and moment detecting system. More particularly, it relates to a method of forming a sensor element to detect forces and/or moments, the sensor element as an article of manufacture, and the sensing apparatus formed with such an element. Most particularly, this invention relates to apparatus useful in detecting and measuring forces and/or moments existing between a robot arm and a tool when the tool encounters resistance to movement in any of a plurality of directions. As used herein, a "moment" is a torque, twist or rotary force acting around an axis in any given direction.
It is a particular object of the present invention to provide a force and/or moment sensing element or unit in which a pair of driving and driven plates are joined together by a plurality of bridging elements to which strain gages or force measuring elements may be readily attached and which may be manufactured more simply and uniformly at a substantially reduced cost. In one embodiment, the unit is fabricated by joining a pair of mounting plates together by a single peripheral ring member. The plates are either separately manufactured, or one may be formed as a cup by including the ring. Each of the plurality of bridge elements is then milled or cut by conventional milling and cutting procedures to carve slots in the ring to leave a plurality of generally circular bridge elements equally spaced apart circumferentially around the periphery as the only connections between the plates. In another embodiment the circular or cylindrical plates are formed with equally spaced axial notches in the periphery and one or more bridge elements are fused or welded in the notches to form an integral unit with the plates spaced apart and parallel to each other. Alternatively, a single cylindrical block may be carved by electron beam erosion to form the equally spaced bridge elements adjacent the periphery of the so-formed plates.
Preferably, the bridge elements are polygonal in shape to provide surfaces substantially perpendicular or angled to the two plates for mounting strain gages or other force sensing devices either on the edges of the bridge element, or on the circular surface of a central hole through the bridge elements.
2. Prior Art
In automatically controlled apparatus, particularly robot systems, it is highly desirable to detect and control operations of the robot arm carrying a work tool. It has been found particularly useful to detect the forces and moments when the tool, carried by the driven side of the arm, encounters a workpiece, so that "feel" may be imparted to the drive mechanism. This is essential to avoid destruction of the arm, tool, or any object (including humans) that is within its range of motion. Additionally, such feel permits the work tool to follow the contour of objects or articles. A particular example is the problem presented by the need to control a deburring or sanding tool to follow the contour of a casting or molding to remove flashing, burrs, and the like for surface finishing. Such tools require continuous and precise measurement of the forces and/or moments to be most effective for versatility and precision in guiding the work arm and tool to produce a fully finished article.
In such a work environment it is also important that the structure measuring the forces be constructed so that the tool will resist contamination by dust or particles generated during grinding, milling, or sanding the workpiece. It is particularly important in robotic manufacturing that the detecting unit continue to measure accurately such forces and moments even with a considerable amount of contamination caused by particles or debris around the sensing elements.
It has been known heretofore to measure forces and moments and, through feedback, to use such measurements to control a drive mechanism. Such results have been obtained through the use of individual measurements of axial forces with standard measuring load cells, such as those shown in Japanese Pat. No. 55-125426, wherein a plurality of pressure-sensitive conductive rubber plates, laminated with electrode plates, measures the direction and intensity of a grasping element.
An article in Finommechanika-Microtechnika (Hungary) Vol. 19, No. 10, October, 1980, discloses a 6-component force meter interposed between the driven and driving members of a robot arm to measure digitally multi-component forces and torques.
U.S. Pat. No. 4,094,182 - Watson et al., discloses driving and driven plates coupled together by three vertical posts equally spaced around the periphery of the unit. Each post carries strain gages. While the unit is useful in detecting forces and moments, it is limited in its overall sensitivity to detect and measure such forces and moments about three mutually perpendicular axes.
Japanese Pat. No. 1145885 discloses a device for measuring forces between a driven member and a driving member. Four cantilever beams at right angles to each other extend radially outwardly from an internal cylinder forming one member. A concentric collar forming the other member is connected to the end of each cantilever beam through three connecting rods mounted in four slots cut axially and as sectors in one end of the concentric collar. Strain gages are mounted on four sides of each radial cantilever beam. Each of the connecting rods and cantilever beams appears to be a separate element requiring multiple assembly steps, similar to the Watson et al. patent.
Japanese Application (Kokai) No. 58-94996 discloses a device for measuring axial torque between two shafts. A center flange and a peripheral ring are connected together through a plurality of radial strain rods. A strain gage is mounted on each rod. The arrangement is also an assemblage of independent elements.
Japanese Application (Kokai) No. 58-205830 discloses a thin-plate force sensor as a substitute for three axial posts between driving and driven members, as in the Watson et al. disclosure. The sensor plate is formed from a circular plate, cut as an asteroid to form six radial cantilever arms. Strain gages are mounted on surfaces of each arm. The individual parts require separate manufacture and assembly.
Japanese Application (Kokai) No. 59-205297 discloses another arrangement of a force detecting device in which a pair of parallel ring plates is connected through four axial pillars and four equally spaced L-shaped beams. The perpendicular sides of the L-shaped beams carry strain gages, and one leg of each "L" is pinned to a driven shaft; the other leg of each "L" is secured to one of the rings driven through the pillars and the other ring. The elements are clearly manufactured separately and then assembled.
In my U.S. Pat. No. 4,488,441, there is disclosed an apparatus for simultaneously measuring perpendicular forces and moments in which equally spaced arches, or horseshoe-shaped units, bridge the space between the two plates. Such generally arch-shaped connecting members are preferably milled from a solid metal block. A center space between the two plates is likewise milled from the block so that the bridging units are integral with the driving and driven plates. To cut the bridging elements in the block in such an arrangement, it is essential that a sector of the plate be cut along mutually perpendicular faces parallel to the common axis of the two plates, so that the faces of the bridging elements are parallel to the unit axis and join the plates along faces radial to the unit axis.
As particularly distinguished from prior art units, the present method provides a simplified method of manufacturing a force-moment sensing unit which is substantially easier to manufacture on conventional machine tools, including automatic lathes, milling machines, drill presses or using electron beam machining and welding. This results in the bridging elements being made integral with the parallel plates so that they are sturdier and more uniformly positioned. Further, the bridging units are so disposed and configured that the mounting of single or multiple strain gages on each bridge is substantially simplified in terms of assembly and reliability. However, most importantly, the greater symmetry of the bridge elements about the circular driving and driven plates of the so produced sensing unit substantially improves performance of the measuring system. In particular, such greater symmetry reduces non-linearities in deflection or strains detected by each bridge when three-dimensional forces or moments are imposed between the plates. Additionally, the structure is better able to resist contamination by particles or debris generated by a robotic work tool such as those fed in welding, grinding, sanding, or cutting operations. This substantially improves and extends the useful life and reduces service necessary to keep a robot arm in operation on a manufacturing or production line to permit 24-hour operation of the robot arm over extended periods of time.